SLATES. 389 



good service. Right or wrong, a thoughtfully - uttered 

 theory has a dynamic power which operates against intel- 

 lectual stagnation ; and even by provoking opposition is 

 eventually of service to the cause of truth. It would, hew- 

 ever, have been remarkable if, among the ranks of geolo- 

 gists themselves, men were not found to seek an explana- 

 tion of slate-cleavage involving a less hardy assumption. 



The first step in an inquiry of this kind is to seek facts. 

 This has been done, and the labors of Daniel Sharpc (the 

 late President of the Geological Society, who, to the loss 

 of science and the sorrow of all who knew him, has so sud- 

 denly been taken away from us), Mr. Henry Clifton Sorby, 

 and others, have furnished us with a body of facts associated 

 with slaty cleavage, and having a most important bearing 

 upon the question. 



Fossil shells are found in these slate-rocks. I have here 

 several specimens of such shells in the actual rock, and oc- 

 cupying various positions in regard to the cleavage planes. 

 They are squeezed, distorted, and crushed ; in all cases the 

 distortion leads to the inference that the rock which con- 

 tains these shells has been subjected to enormous pressure 

 in a direction at right angles to the planes of cleavage. 

 The shells are all flattened and spread out in these planes. 

 Compare this fossil trilobite of normal proportions with 

 these others which have suffered distortion. Some have 

 lain across, some along, and some oblique to the cleavage of 

 the slate in which they are found ; but in all cases the dis- 

 tortion is such as required for its production a compressing 

 force acting at right angles to the planes of cleavage. As 

 the trilobites lay in the mud, the jaws of a gigantic vice ap- 

 pear to have closed upon them and squeezed them into the 

 shapes you see. 



We sometimes find a thin layer of coarse, gritty mate- 

 rial, between two layers of finer rock, through which and 

 across the gritty layer pass the planes of lamination. The 



